1. Field of the Invention
The present invention relates to a method of heat treating metallic workpieces using a continuous-heating furnace, a pusher-type furnace, or a gravity-discharge furnace having at least two treatment chambers through which the workpieces successively pass; the chambers, which are provided with doors, serve to respectively heat treat a plurality of workpiece charges, with individual ones of said charges being cyclically received by the respective chambers, being conveyed through the latter while heat treatment is undertaken, and subsequently being released by the chambers. The present invention also relates to an apparatus for carrying out this method, especially for the two-stage carburization of metallic workpieces in a controlled atmosphere, and has furnace chamber means which is divided into several treatment chambers having different temperatures and/or furnace atmospheres, and which is provided with a door for loading the workpieces into the furnace chamber means and with the door for withdrawing the workpieces therefrom. A supercarburization is preferably undertaken in the first treatment chamber with a high carbon potential, and a diffusion-decarburization, which leads to the final desired values for the case-hardening depth, is carried out in the second treatment chamber.
2. Description of the Prior Art
Up to now, only gravity-discharge or pusher-type furnaces have been used for this purpose. The drawback with these furnaces is that the treatment time for the individual charges cannot be varied. Each charge which is introduced into the furnace leaves the furnace in an unchangeable sequence after a predetermined period of time. As a result, these furnaces are limited in their application, especially for the treatment of large number of similar workpieces which have to undergo a varied or identical treatment.
Rotary hearth furnaces are also known for heat treating metallic workpieces. With these furnaces, workpieces are introduced through a loading door onto a circular hearth; the workpieces pass through the furnace along a long path in a specific direction of rotation, and after termination of the heat treatment are again removed in the same sequence in which they were loaded (U.S. Pat. No. 4,412,813 Wulf issued Nov. 1, 1983). Such rotary hearth furnaces also operate pursuant to the known gravity-discharge or continuous-heating principle, pursuant to which the retention time of all of the workpieces in the individual stations (treatment chambers) is the same. A further drawback of these heretofore known rotary hearth furnaces is that the furance chamber cannot be divided into a plurality of zones, which for example have different carbon potentials, so that in a practical application, for example for carburization, rotary hearth furnaces can only be used to achieve a case-hardening depth of at most 0.6 mm, since greater case-hardening depths cannot be achieved, or at least cannot be achieved in an economical manner.
An object of the present invention is to provide a method as well as a continuous-heating furnace for heat treating metallic workpieces, whereby, while avoiding the aforementioned drawbacks, it is possible to simultaneously treat charges which require different types and/or duration of treatment, especially for achieving different case-hardening depths during carburization in a two-stage method, while at the same time fully utilizing the capacity of the furnace. The method and apparatus should provide great flexibility in the application of the furnace for simultaneously treating workpieces having varying sizes, shapes, and quantities.